On Mon, Feb 15, 2016 at 8:23 PM, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote: > On Mon, Feb 15, 2016 at 07:03:33PM +0000, Marc Zyngier wrote: >> On 15/02/16 18:54, Rafael J. Wysocki wrote: >> > That would explain it, thanks. >> > >> > So it looks like we should always use irq_work_queue() on UP even if >> > CONFIG_SMP is set, shouldn't we? >> >> Something like that, yes. CONFIG_SMP is not an indication of an SMP >> system anymore (we've even dropped the config option on arm64). >> >> Hopefully num_possible_cpus() is reliable enough to let you do the right >> thing... > > CONFIG_SMP just says whether to include support for SMP. It doesn't > mandate running on a SMP system. :) > > I've been looking around the usages of irq_work_queue_on in kernel/ > in -rc4, and some places seem to check for "this CPU": > > /* > * It is possible that a restart caused this CPU to be > * chosen again. Don't bother with an IPI, just see if we > * have more to push. > */ > if (unlikely(cpu == rq->cpu)) > goto again; > > /* Try the next RT overloaded CPU */ > irq_work_queue_on(&rt_rq->push_work, cpu); > > I'm not sure about tell_cpu_to_push(). > > It's also called via tick_nohz_full_kick_cpu(), and the core scheduler > avoids calling this for the current CPU: > > if (tick_nohz_full_cpu(cpu)) { > if (cpu != smp_processor_id() || > tick_nohz_tick_stopped()) > tick_nohz_full_kick_cpu(cpu); > > I'm not sure about add_nr_running() in kernel/sched/sched.h - I think > that _could_ be a problem even without Rafael's cpufreq change. > > So... the question is what do we do with irq_work_queue_on() in general > when called on non-SMP systems. I guess it might fall back to arch_irq_work_raise() when asked to queue on the same CPU, so long as that will always do the right thing (ie. actually queue on the same one). Thanks, Rafael -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html